Identification of an odorant-binding-protein-like sequence in Amblyomma americanum (Acari: Ixodidae)

Ticks use chemosensory reception to identify hosts and mates, but so far no olfaction-related hydrophobic ligand-binding proteins, of the type that capture odorants and pheromones in other terrestrial animals, have been fully characterized.  We now report on a proteomics analysis of the Amblyomma americanum fore-tarsus, which includes the chemosensing Haller's organ.  We detected a protein fragment that has a high degree of sequence similarity to proteins of unknown function in Ixodes scapularis, Rhipicephalus microplus, and Metaseiulus occidentalis, but no statistically significant sequence similarity to any proteins in other arthropods.  Three-dimensional homology modeling showed significant similarities between the full-length I. scapularis protein and twelve insect odorant or pheromone-binding proteins (OBP family).  Hence, we will refer to the protein as Isca-OBPL (I. scapularis OBP-like). Isca-OBPL contains six cysteine residues, four of which match positions of cysteines that form disulfide bonds in typical insect OBPs, and two that are in unique positions. The 3D model suggests that Isca-OBPL lacks the same disulfide that is missing in the so-called C-minus OBPs, and it likely has a novel disulfide between the unique cysteines, resembling in a general way the so-called C-plus OBPs. The model indicates a central hydrophobic pocket, which could function as a ligand binding site. Future studies will test whether Isca-OBPL and related proteins are involved in acarine olfaction. The U.S. Department of Agriculture is an equal opportunity provider and employer. This research was funded in part by USDA-ARS appropriated project 3094-32000-039-00D